Toward the Understanding of Modulation in Molecular Materials

Article pubs.acs.org/crystal

Toward the Understanding of Modulation in Molecular Materials: Barluenga’s Reagent and its Analogues. Yejin Kim, Emma J. Mckinley, Kirsten E. Christensen,* Nicholas H. Rees, and Amber L. Thompson Chemical Crystallography, Department of Chemistry, University of Oxford, OX1 3TA Oxford, U.K. S Supporting Information *

ABSTRACT: Investigations into the phase transition of Barluenga’s reagent revealed a transient incommensurately modulated phase; the structures are presented herein. To understand the origin of the modulated phase and the chemistry that can affect it, analogues of Barluenga’s reagent were synthesized and studied. In this context, the halogen and anion can easily be exchanged. Studying different analogues led to the development of the Ratchet Model to describe the behavior in the solid state leading to a better understanding of modulation in this class of molecular crystal structure.

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inorganic materials.11 Molecular modulated materials are found within the literature5 and especially structures like the ureainclusion compounds have drawn great interest.12,13 Generally modulation in molecular materials are now seen more often with higher intensity sources being used. In our experience the ability to reconstruct precession images means that more molecular materials can be recognized as modulated materials rather than heavily disordered compounds in the solid state. Bis(pyridine)iodonium(I) tetrafluoroborate, IPy2BF4, otherwise known as Barluenga’s reagent is an iodinating and oxidizing reagent known to have a solid state phase transition.14 The synthesis and the use of IPy2BF4 was first reported by Barluenga in 198515 and the structure was first reported by Á lvarez-Rúa et al. at 120 K.16 In 2003, Chalker et al. reported an alternative synthesis consisting of two steps.14 They reported a solid state phase transition occurring in Barluenga’s reagent resulting from the BF4− ion becoming more ordered on cooling. The space group at 250 K was reported in C2/n17 with half the molecule in the asymmetric unit, and the BF4− disordered around a 2-fold rotation axis. In contrast, the space group at 150 K was found to be P21/n with a single whole molecule and two half molecules balanced by two BF4− counterions in the asymmetric unit. The loss of inversion centers forces the cell to double resulting in the iodine now being on a general position. Here, we present the results of our investigations into this phase transition and the discovery of a transient modulated intermediate phase. In an effort to understand how changes to the chemistry affect the phase changes in Barluenga’s Reagent we have synthesized and studied a range of analogous compounds varying the anions and modifying the cation.

INTRODUCTION Single crystal X-ray diffraction is a maturing technique. However, with the advent of increased access to higher intensity sources (for example, microfocus X-ray sources and synchrotron radiation) and increasingly sensitive detectors (including the latest generation of CCDs and the new photon counting detectors), it has become apparent that there are often features lurking within the data that we have previously been able to ignore.1,2 The study of these features, including diffuse, non-Bragg scattering,3,4 and satellite reflections arising from modulation5 represent the new frontiers of modern crystallography. Modulation describes periodic distortions to the conventional three-dimensional, periodic structure and therefore can affect our understanding of the physical properties of the crystal. The superspace approach was developed in the 1970s by Janner, Janssen and de Wolff following the 9th International Crystallography Conference in Kyoto (1973), to solve the incommensurately modulated structure of sodium carbonate.6 In fact, the structure of sodium carbonate was the first to be refined using one modulation wave and first order satellites.7 In modulated structures, there are periodic distortions that arise when atoms, groups of atoms, or even whole molecules are shifted or rotated with respect to their neighbors following a rule which can be described mathematically using a Modulation Function.8 This additional periodicity in the distortions (modulations) causes satellite reflections around the main reflections in a diffraction pattern (Figure 1). Incommensurately modulated phases can be found in many important solid state materials and transitions to the incommensurate modulated structure corresponds to a change of key physical properties.9,10 For this reason, it is important to understand the structure of incommensurately modulated phases to comprehend the mechanism of the transition and properties associated with the modulated state.5,8 Modulation in molecular materials is a newer phenomenon, as much of the interest in modulated structures has been in © XXXX American Chemical Society

Received: July 3, 2014 Revised: October 30, 2014

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dx.doi.org/10.1021/cg500983s | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Figure 1. Schematic representation of hypothetical crystal structures and their diffraction patterns. (a) The unit cell (highlighted by the red box) is replicated throughout, giving a diffraction pattern of reflections at equal distances. (b) In addition to the original set of reflections shown in panel a, there are weaker reflection in between, because of the doubling of the unit cell. (c) Tripling of the unit cell gives rise to two weaker reflections between the original set of reflections. These reflections are weaker because the main feature of the unit cell is still in place and it is just the side groups moving. Both panels b and c can also be described as commensurately modulated structures, which can be described as a 2-fold or 3-fold superstructure, respectively. (d) The displacement of molecules can be described using a wave function and this gives rise to satellite reflections.

Figure 2. Reconstructions of the hk0 reciprocal lattice layers of the IPy2BF4 cell at (a) 300, (b) 210, and (c) 150 K. The photographs are centered on (−3, −1.5, 0) for a clear view. (a) At 300 K, there are no satelite reflections which appear in panels b and c. Red boxes show the zigzag pattern in panel b at 210 K, which straightens out in panel c at 150 K. These correlate with the observation of two phase transitions, one between (a) 300 and (b) 210 K and the other between (b) 210 and (c) 150 K.

Figure 3. Differential scanning calorimetry of Barluenga’s Reagent, IPy2BF4. On both cooling (blue) and heating (red), discontinuities in the heat flow were seen at 193 K and a broad peak above 250 K.

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RESULTS AND DISCUSSION Barluenga’s Reagent, IPy 2BF 4. In an attempt to determine the phase transition temperature of Barluenga’s Reagent single crystal X-ray data were collected at 300, 250,

210, 150, and 100 K. Upon inspection of the reconstructed reciprocal lattice layers at the different temperatures (Figure 2), a transient modulated phase was observed. This was characterized by satellite reflections which appeared in a zigzag B



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pattern indicating that the satellite reflections are incommensurate with the main lattice with a q-vector longer than 0.5. Upon cooling, these satellite reflections become conventional Bragg reflections that can be indexed with a unit cell doubled along b at 150 K, or described in another way the satellite reflections are now commensurate with the main lattice with a q-vector of 1/2. The presence of this third phase was confirmed by the differential scanning calorimetry (DSC) results (Figure 3). Thus, the modulated phase is believed to persist between 250 and 193 K on cooling. Above this range, Barluenga’s Reagent is thought to exist in the disordered high temperature phase, and below this range, there is a low temperature ordered phase. Solid-state variable temperature 13C NMR however, only showed subtle changes due to the phase transition below which we start to resolve unequivalent rings (Figure 4).

Figure 5. Structure of Barluenga’s Reagent, IPy2BF4 in (a) the high temperature phase at 250 K, (b) the modulated phase at 210 K, and (c) the low temperature phase at 150 K. In panel a, BF4− ions are disordered and IPy2+ ions align parallel to each other at 250 K. In the low temperature phase panel c, BF4− ions are ordered and alternate between being up and down, followed by tilting of alternate IPy2+. In panel b, BF4− are ordered, but the structure has not settled down to the low temperature phase. Deviations from panel c are highlighted in red.

Figure 4. Solid-state 13C CP MAS NMR VT experiment results for IPy2BF4. Three peaks at 323 K each represent ortho, para and meta carbon environments with respect to the nitrogen, in the order of high to low chemical shifts. The peaks start to split from 255 K.

The structure of the modulated phase at 210 K was solved, refined and compared with the high temperature phase (250 K) and low temperature phase (150 K, Figure 5). The high temperature phase at 250 K has disordered BF4− ions with three components located around an inversion center. In addition, IPy2+ ions are all aligned parallel to each other. On cooling, the BF4− ions become ordered in the low temperature phase. As a result the BF4− anions alternate between being up and down, followed by alternate IPy2+ cations tilting to fit themselves into the tighter packing. The modulated phase at 210 K shows what happens during the phase transition. BF4− ions, instead of following the strict up and down alternation seen at 150 K, show deviations which are marked with red circles. Also, the tilting of IPy2+ is less pronounced and the angle depends on the position of BF4− (up or down). Anion Substitution: Analogues of Barluenga’s Reagent. Different anions were substituted in the synthesis of Barluenga’s reagent in order to investigate how the modulated phase would be affected by changes in the chemistry of the molecule. Exchange of the BF4− anion with the PF6− anion revealed a different structure that did not have a phase transition upon cooling.18,19 The PF6− anion has an octahedral shape and is about 18% larger in radii than the tetrahedral shaped BF4− anion.20 Other anion analogues of Barluenga’s Reagent were synthesized namely triflate (OTf−),21 nitrate (NO3−)22 and I3− (with excess iodine of crystallization), as

reported previously.23 In general, structures were checked at high and low temperature to investigate the possibility of a phase transition and their structures were solved. None of the compounds showed a modulated phase and their structures were different to the original Barluenga’s Reagent (Figure 6). This made it interesting to study ClO4−, as this anion has the same shape as BF4−, but the size is slightly larger. For the perchlorate analogue at room temperature the structure indexes with the same cell as IPy2BF4 and we also observe the modulation, though weaker than in the original Barluenga’s Reagent. On the other hand the onset of the transient modulated phase starts at 280 K and does not transform until well below 100 K, which means that the modulated phase of IPy2ClO4 exists over a broader temperature range than its analogue IPy2BF4. However, the structures are isostructural and isomorphous with only a small difference between the anions.24 VT solid-state NMR experiments were also carried out (Figure 8) to study the dynamics of the transition. From 323 to 289 K, three single peaks were observed corresponding to the ortho, para, and meta carbons. These peaks started splitting into multiple peaks from 255 K, indicating that different ortho, para, and meta carbons which were equivalent, become C



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Figure 6. Structure of the IPy2+ salts: IPy2PF6 viewed down the (a) b axis and the (b) c axis, IPy2OTf viewed down the (c) a axis and (d) [011] direction and IPy2I3·2I2 viewed down the (e) a axis and (f) c axis.

Bromine Analogues of Barluenga’s Reagent. It is not only the anion that can be modified, the iodine in the center of the cation can also be replaced. Changing the iodine to bromine made the compounds a lot less stable, and the reaction was very sensitive to any moisture present during synthesis. The following analogues were synthesized: BrPy 2 BF 4 , BrPy2PF6, BrPy2ClO4 and BrPy2OTf, but no modulated phase was observed for any of the compounds. BrPy2OTf is the same as previously reported.21 Initially, the iodine and bromine analogues were expected to be isomorphous because of the small size difference, but this was not the case and changing the central halogen appeared to have an even larger effect than changing the anion (Figure 10). This was surprising given that (superfically at least) the change would appear to be subtle with only a slight lengthening of the cation. BrPy2PF6 exhibited a structure related to, but more complex than, that for IPy2PF6. Similarly, BrPy2BF4 was found to have a structure related to that of IPy2BF4, but with each layer doubled. BrPy2ClO4 on the other hand exhibited a completely different structure to its iodine analogue. The layers form sandwiches and the cation shows a slight twisting and bending. The fact that IPy2+ salts of BF4− and ClO4− were isomorphous while the BrPy2+ salts were not, suggests that the packing is dependent on the ratio of the size of anion and the halogen. HalPy2OTf salts were found to be an exception where the iodine and bromine analogues were isomorphous with quite similar dihedral and bending angles for the cation. For the triflate salts, the size of the anion appeared to dominate the packing. Perhaps the most striking result is the dihedral angle seen in the cation of BrPy2BF4. In the majority of the structures the

inequivalent on cooling. However, the average peak positions stay the same, suggesting a transition to a phase with more dynamic character (i.e., the modulated phase) rather than to a phase with a completely different structure. The Ratchet Model. The driving force behind the phase transitions observed for IPy2BF4 and IPy2ClO4 is believed to be the ordering effect of the anion in the packing. The high temperature phase for both IPy2BF4 and IPy2ClO4 were found to be disordered. At high temperatures, the BF4− ions tumble freely yielding the high temperature dynamic structure. Upon cooling, BF4− ions move more slowly due to the decrease in the energy available. Eventually, the anions settle down in the orientation and arrangement that they are most stable in, which is now the ordered stable structure. However, at lower temperatures, the shape of the space available for BF4− anion changes slightly and IPy2+ adjusts for a better fit to give a tighter packing. To describe the ordering of the anions and the distortions it causes on cooling we introduced the Ratchet Model. A ratchet is a device consisting of a pawl and a gear that allows a rotation in one direction (Figure 9). In these salts, the rigid rod-like cation HalPy2+ acts as the pawl, while the anion acts as the gear. When the gear (anion) tumbles, the pawl (cation) is directly affected and this effect is propagated throughout the structure. In IPy2BF4, the rotation of one anion causes the IPy2+ pawl to move. This dictates the direction the neighboring BF4− moves which in turn affects the neighboring pawl which is unable to tilt back to a more upright orientation, because of the limited space available. This description is equally applicable to the modulated phase in IPy2ClO4. D



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Figure 9. Schematic of a simple mechanical ratchet (left) with a representation of how the “Ratchet Model” applies to Barluenga’s Reagent (right). In the mechanical device, the shape of the cogs ensures the gear can only rotate in one direction. In the crystal structure, the BF4− anions behave like gears and IPy2+ cations act as a double pawl, with the pivot in the center. Rotation of one anion causes the IPy2+ pawl to move which dictates the direction the neighboring BF4− moves and thus propagates the motion through the crystal.

with the dihedral angles greater than 10° were hard to crystallize, but BrPy2BF4 with the largest dihedral angle was the most difficult compound to crystallize and the dihedral angle appeared to be strongly related to the issues with crystallizing these compounds.

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CONCLUSIONS In addition to the previously reported phase transition in Barluenga’s Reagent, IPy2BF4, we have demonstrated that the symmetry allowed transition proceeds via a transient modulated phase. Replacing the anion to give IPy2ClO4 revealed a similar transition, however, the modulated phase is persistent over a larger temperature range. The driving force behind these phase transitions is believed to be the ordering of the anion within the crystal structure. The anions are dynamically disordered in the high temperature phase for both IPy2BF4 and IPy2ClO4. Upon cooling, the structure changes via a modulated phase to a completely ordered low temperature phase. This behavior is associated with the unit cell contraction, a reduction in kinetic energy of the tumbling anion and an ordering of the anion and cation. These changes to the tumbling anion and long flat cation can be described with the proposed Ratchet Model. A number of other analogues were synthesized replacing both the anions and the halogen in the center of the HalPy2+ cation, but in general the structures were all different and, in accordance with the ratchet analogy, none of them exhibited an order−disorder phase transition.

Figure 7. Structure of IPy2ClO4 in (a) the high temperature phase at 300 K, (b) the modulated phase at 250 K, (c) the low temperature phase at 30 K. In panel a, ClO4− ions are disordered and IPy2+ ions align parallel to each other at 300 K. In the low temperature phase (c), ClO4− ions are ordered and alternate between being up and down, followed by tilting of alternate IPy2+. In panel b, ClO4− are ordered, but the structure has not settled down to the low temperature phase. Deviations from panel c are marked in red circles.

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EXPERIMENTAL METHODS

Synthesis. The variants of Barluenga’s reagent studied in this project were synthesized using analogous chemistry. A general procedure was to work in anhydrous conditions, as halogens tend to get reliberated in moisture.25 In all the cases, silver salt (AgX where X = desired anion) or the silver analogue (AgPy2X) were reacted with the halogen (and pyridine for AgX), which replaces the silver cation in the structure, precipitating out silver halide as the side product. The tetrafluoroborates, IPy2BF4 and BrPy2BF4 were synthesized following the synthesis proposed by Chalker et al.14 For the bromine analogue of Barluenga’s reagent, the synthesis was carefully followed using bromine instead of iodine.26 Synthesis of the hexafluorophosphates and perchlorates analogues of Barluenga’s Reagent was also a two-step synthesis, using NaX or KX and AgNO3 to make AgPy2X, which is then reacted with halogen to give the product. Homsi et al. proposed a synthesis of bis(2,4,6trimethylpyridine)halogen hexafluorophosphate in 2000.27 This

Figure 8. Solid-state 13C CP MAS NMR VT experiment results for IPy2ClO4. Three peaks at 323 K each represent ortho, para, and meta carbon environments with respect to the nitrogen, in the order of high to low chemical shifts. The peaks start to split from 255 K.

pyridine rings in the HalPy2+ are approximately planar, in BrPy2BF4 however, the dihedral angle is 65.69(12)°.The dihedral and bending angles of HalPy2+ appear to be correlated with the stability, as well as the packing. The compounds with nonzero dihedral angles were more difficult to crystallize and were also found to decompose more quickly: the triflate salts, E



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Figure 10. Structure of the BrPy2+ salts: (a) BrPy2OTf, (b) BrPy2PF6, (c) BrPy2BF4, (d) BrPy2ClO4. The orientation of the structures are chosen to give the best view for each of the four structures.

Figure 11. Differential scanning calorimetry of Barluenga’s Reagent, IPy2ClO4. Upon both cooling (blue) and heating (red), discontinuities in the heat flow were seen around 280 and at 230 K. IPy2ClO4: m.p. 148−156 °C; IR 1602, 1454, 1206, 1085, 1055, 1010, 758, 688, 637, 621 cm−1; 1H NMR (400 MHz, CDCl3) δ 8.87 (d, J = 5.5 Hz, 4H), 8.18 (t, J = 8.0 Hz, 2H), 7.62 (t, J = 6.8 Hz, 4H). IPy2OTf: m.p. 82−87 °C; IR 3031, 1600, 1455, 1262, 1219, 1151, 1064, 1026, 1007, 775, 752, 690, 633 cm−1; 1H NMR (400 MHz, CDCl3) δ 8.90 (dd, J = 4.5, 2.7 Hz, 4H), 8.18 (td, J = 7.8, 1.7 Hz, 2H), 7.63 (dd, J = 7.7, 5.9, 4H). BrPy2PF6: m.p. 126−134 °C; IR 1603, 1458, 1208, 1066, 1040, 1011, 823, 765, 690, 638 cm−1; 1H NMR (400 MHz, CDCl3) δ 8.83− 8.76 (m, 4H), 8.20 (t, J = 7.7 Hz, 2H), 7.74 (t, J = 6.7 Hz, 4H). BrPy2ClO4: m.p. 106−115 °C; IR 1601, 1457, 1206, 1081, 1062, 1038, 1009, 777, 752, 683, 638, 619 cm−1; 1H NMR (400 MHz, CDCl3) δ 8.84 (s, 4H), 8.17 (t, J = 7.9 Hz, 2H), 7.72 (t, J = 7.2 Hz, 4H).

procedure was repeated substituting (2,4,6-trimethylpyridine) with pyridine and using bromine in the final step to make BrPy2PF6. IPy2ClO4, and BrPy2ClO4 were synthesized using the same method. This was repeated using bromine to obtain BrPy2ClO4 after many different crystallization attempts. To synthesize the nitrate and triflate analogues of Barluenga’s Reagent then AgX is directly reacted with pyridine and halogen in one step to yield the product following the synthesis reported by Neverov et al. in 2002.28 This synthesis was carefully followed for both IPy2OTf and BrPy2OTf. IPy2BF4: m.p. 151−168 °C; IR 1600, 1453, 1032, 753, 698 cm−1; 1H NMR (400 MHz, CDCl3) δ 8.61 (d, J = 4.3 Hz, 4H), 7.68 (t, J = 7.8 Hz, 2H), 7.36−7.20 (m, 4H). IPy2PF6: m.p. 162−167 °C; IR 1602, 1456, 1207, 1060, 1041, 1010, 880, 824, 758, 687, 637 cm−1; 1H NMR (400 MHz, CDCl3) δ 8.82 (d, J = 5.4 Hz, 4H), 8.18 (t, J = 7.7 Hz, 2H), 7.65−7.57 (m, 4H). F



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BrPy2OTf: m.p. 80−82 °C; IR 3096, 1539, 1457, 1261, 1221, 1152, 1025, 997, 776, 753, 685, 624 cm−1; 1H NMR (400 MHz, CDCl3) δ 8.83−8.76 (m, 4H), 8.20 (t, J = 7.7 Hz, 2H), 7.74 (t, J = 6.7, 4H). Variable-Temperature 13C NMR. Variable-temperature 13C NMR was carried out on IPy2BF4 and IPy2ClO4 to investigate the structural-phase transition that was observed in the single-crystal X-ray diffraction experiments. 13C CP MAS NMR spectra were obtained on a Bruker Avance III HD spectrometer equipped with a 9.4 T magnet, operating at 399.9 MHz for 1H and 100.6 MHz for 13C using 4 mm O.D. rotors containing 100 mg of sample and a MAS rate of 10 kHz utilizing dry nitrogen for all gas requirements. A cross-polarization sequence with a variable X-amplitude spin-lock pulse29 and spinal64 proton decoupling was used. 250 transients were acquired using a contact time of 3 ms, an acquisition time of 34 ms (2048 data points zero filled to 24 K) and a recycle delay of 20 s. For the variable temperature experiments nitrogen gas was passed through a heat exchanger immersed in liquid nitrogen prior to passing over a heater element followed by the sample, which was allowed to equilibrate for 20 min upon reaching the desired temperature before tuning the probe and acquiring the spectrum. The actual sample temperature was calculated from the indicated temperature by observing certain distinct phase changes in known compounds (camphor, DABCO, adamantane).30 All 13C spectra were referenced to adamantane (the upfield methine resonance was taken to be at δ = 29.5 ppm31 on a scale where δ (TMS) = 0) as a secondary reference. Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) was recorded on a PerkinElmer Pyris 1 Differential Scanning Calorimeter operated using the Pyris software. Liquid nitrogen was used as the internal coolant operated automatically by a PerkinElmer CryoFill liquid nitrogen cooling system. Helium was used as a purge gas. The samples were placed in a standard aluminum pan, and an empty pan was used as reference. The calorimeter was calibrated using an internal reference of cyclohexane. DSC measurement for IPy2ClO4 shows two discontinuities (Figure 11; Table 1) at 280 and 230 K. However, from X-ray data (Figure S2

Hydrogen atoms were not generally provided by the initial solution, however they were usually clearly visible in the difference Fourier map. As these structures only included hydrogen atoms bound to aromatic carbon atoms these were positioned at geometrically sensible positions and refined using soft restraints prior to inclusion in the refinement using a riding model.36 Refinement of the Modulated phase of Barluenga’s Reagent, IPy2BF4 and IPy2ClO4. For the two incommensurately modulated structures IPy2BF4 and IPy2ClO4, only first order satellite reflections were observed. Both structures were solved with Superflip34 and refined using JANA200637 with only one modulation function used for each atom. The data collected at 210 K for IPy2BF4 were found to be incommensurately modulated with a q-vector of (0, 0.5622, 0). Upon inspection of the systematic absences, the spacegroup was determine to be C2/c (0β0)s0 with a monoclinic unit cell of a = 12.3353(13) Å, b = 7.9515(8) Å, c = 14.7011(16) Å, and β = 113.236(10)°. The cation IPy2+ was refined with positional modulation whereas the anion BF4− was refined with an occupational modulation, not the Crenel function, but a smooth occupancy modulation that takes into account some disorder. This was carefully checked by refining the structure with and without the Crenel function and the refinement was significantly better without. The structure can be seen in Figure 5. At first it was believed that the transient modulated phase could be a lock-in phase in which case a change of length of the q-vector should be observed on cooling from the high temperature phase through the modulated phase to the fully ordered phase. Further studies collecting data at different temperature intervals both on cooling and heating confirms minimal change to the q-vector upon cooling and an abrupt change into the low temperature superstructure (see Figure 12). There are minor variations of the q-vector length upon heating compared to those seen upon cooling, which appears to be due to hysteresis.

Table 1. Phase Transition Temperatures from DSC compound

1st transition (K)

2nd transition (K)

IPy2BF4 IPy2ClO4

290−250 280 (230)

195 below 100

in Supporting Information) a gradual transition is observed from the high temperature disordered phase to the modulated phase starting at 280 K. Although currently we cannot fully explain the inconsistencies seen in the transition temperature, the number of satellite peaks indexed increases significantly below 230 K. Single-Crystal X-ray Diffraction. Single-crystal X-ray diffraction data were collected using an Oxford Diffraction (Agilent) SuperNova diffractometer fitted with an Oxford Cryosystems Cryostream 700 Plus open flow nitrogen cooling device. The CrysAlisPro software was used for data collection and integration. For the variable-temperature (VT) data collections, CrysAlisPro was used to control the temperature regime. In general a cooling rate of 120 K/h was used and the temperature was allowed to stabilize for 5−10 min before the data collection started. For weakly diffracting samples and/or low temperature data collection below 90 K, Diamond Light Source, Beamline I19 was used.32 Data were collected using a Rigaku CrystalLogic Kappa goniometer with a Saturn 724+ CCD detector with a X-ray energy at the zirconium absorption edge (λ = 0.6889 Å). CrystalClear was used for data collection and initial unit cell determination, whereas data reduction and numerical absorption corrections were applied with CrysAlisPro software. The diffractometer was fitted with either an Oxford Cryostream Plus33 or HeliX. Structure Solution and Refinement. In general, structures were solved using SuperFlip34 within the CRYSTALS suite35 or refined from a known isomorphous starting model. The structures were then modified, improved and optimized by full-matrix least-squares on F2.

Figure 12. Variation in length of q-vector in IPy2BF4 on cooling (blue) and heating (red). The lines are drawn to guide the eye. From 230 to 300 K, IPy2BF4 exists as a high temperature nonmodulated phase.

The data for IPy2ClO4 were collected at 250 K using I19 (EH1) at Diamond Light Source. 32 They were similarly found to be incommensurately modulated with a q-vector of (0, 0.5576, 0). Upon inspection of the systematic absences the space group was determined to be the same as IPy2BF4, C2/c (0β0)s0 with a monoclinic unit cell of a = 12.4268(8) Å, b = 8.0795(5) Å, c = 14.8366(12) Å, and β = 113.271(7)°. The cation IPy2+ was refined with positional modulation whereas the anion ClO4− was refined with a crenel function as occupancy modulation. G



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(18) Okitsu, T.; Yumitate, S.; Sato, K.; In, Y.; Wada, A. Chem.Eur. J. 2013, 19, 4992−4996. (19) The structure of IPy2PF6 has been reported in C2, but following careful examination of our data it was determined to be C2/m in our case. (20) Roobottom, H. K.; Jenkins, H. D. B.; Passmore, J.; Glasser, L. J. Chem. Educ. Inorg. Chem. 1999, 76, 1570−1573. (21) Neverov, A. A.; Feng, H. X.; Hamilton, K.; Brown, R. S. J. Org. Chem. 2003, 68, 3802−3810. (22) IPy2NO3 formed yellow crystals, but they were found to suffer such severe radiation damage that it was impossible to study the crystal structure. (23) Hassel, O.; Hope, H. Acta Chem. Scand. 1961, 15, 407−416. (24) ClO4− has a radius of 0.225 ± 0.019 nm and its volume is 12.3 ± 3.5% bigger than BF4− in MX salts,20,38 however, the volume of the two anions were reported to be much more similar by Yamada et al.39 (54.4 Å3 for ClO4− and 53.4 Å3 for BF4−). (25) Lemieux, R. U.; Morgan, A. R. Can. J. Chem. 1965, 2190−2197. (26) BrPy2BF4 was very unstable and decomposed before we could measure melting point, IR, and 1H NMR. (27) Homsi, F.; Robin, S.; Rousseau, G. Org. Synth. 2000, 77, 206− 211. (28) Neverov, A. A.; Feng, H. X.; Hamilton, K.; Brown, R. J. Org. Chem. 2003, 3802−3810. (29) Peersen, O.; Wu, X.; Kustanovich, I.; Smith, S. J. Magn. Reson. 1993, A, 34−339. (30) Riddell, F. G.; Spark, R. A.; Gaijnther, G. J. Magn. Reson. 1996, 34, 824−828. (31) Earl, W. L.; Vanderhart, D. J. Magn. Reson. 1982, 48, 35−54. (32) Nowell, H.; Barnett, S. A.; Christensen, K. E.; Teat, S. J.; Allan, D. R. J. Synchrotron Radiat. 2012, 435−441. (33) Cosier, J.; Glazer, A. M. J. Appl. Crystallogr. 1986, 19, 105−107. (34) Palatinus, L.; Chapuis, G. J. Appl. Crystallogr. 2007, 40, 786− 790. (35) Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, K.; Watkin, D. J. J. Appl. Crystallogr. 2003, 36, 1487. (36) Cooper, R. I.; Thompson, A. L.; Watkin, D. J. J. Appl. Crystallogr. 2010, 43, 1100−1107. (37) Petricek, V.; Dusek, M.; Palatinus, L. Z. Kristallogr. 2014, 345− 352. (38) Jenkins, H. D. B.; Roobottom, H. K.; Passmore, J.; Glasser, L. Inorg. Chem. 1999, 38, 3609−3620. (39) Yamada, M.; Hagiwara, H.; Torigoe, H.; Matsumoto, N.; Kojima, M.; Dahan, F.; Tuchagues, J.-P.; Re, N.; Lijima, S. Chem. Eur. J. 2006, 12, 4536−4549.

ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre (CCDC 1030913−1030929) and copies of these data can be obtained free of charge via www. ccdc.cam.ac.uk/data_request/cif. Also present are reconstructed precession images of IPy2ClO4 at 250 and 30 K and at 300, 290, 280, 270, 250, and 100 K; list of unit cells for all studied compounds; illustration of occupancy functions for IPy2BF4 and IPy2ClO4; 2D Fourier maps of selected atoms in IPy2BF4 and IPy2ClO4 along the forth dimension; characterization data for all new compounds including structural data in CIF format; and for the two incommensurately modulated structures IPy2BF4 and IPy2ClO4, a (3 + 1)D CIF and an approximation as a 10-fold superstructure along the b-axis for drawings in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +44 (0) 1865 85023. Fax: +44 (0)1865 285021. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are very grateful to Prof. Ben Davis and Dr. Justin Chalker for helpful discussions. We also gratefully thank Diamond Light Source and the beamline scientists on beamline I19 for an award of beamtime (MT7768). KEC would like to thank The Violette and Samuel Glasstone Fund for a fellowship award and Jesus College, Oxford, for support. The EPSRC for funding the solid state NMR spectrometer.

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REFERENCES

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